Rice husk (RH) is a major agro-waste in rice-producing countries and its management is a serious concern. RH biochar (RHB) is generally used for soil amelioration; however, its functionality can be further enhanced by tailoring its properties to meet the specific requirement of crops. Herein, we report that the conversion of RHB to its nanoform (hereafter termed as NRB) and employing surface engineering could enable its use as a slow-release carrier for the delivery of micronutrients to plants. We used Zn , a key plant micronutrient, to study its effect in rice seedling growth. The results showed that the Zn loaded functionalized biochar (Zn-FRB) contributed to the enhanced root and shoot growth of rice compared to that of NRB. Zn-FRB at a high concentration (100 µg mL-1) showed ~11% increase in the soluble protein content. Zn-FRB (30 and 200 µg mL-1) also showed reduction in antioxidative enzyme activity (CAT and APX) compared to that of NRB suggesting an imperative role of Zn in protecting against oxidative damage of membrane lipids. Zn-FRB application caused a change in expression of Zn homeostasis genes and phloem transporter gene and increased their transcript levels as revealed by qPCR studies. According to these results, Zn-FRB was found to be more effective than NRB in enhancing the plant growth, reducing the oxidative damage and transportation of Zn. These results suggest the potential of this approach as new slow delivery system of micronutrients to plants.

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